A big thumbs up and thank you to the Bootstrap team.

Building Bootstrap 4 from source (which is what you want to do for your own themes and customizing) requires a somewhat hefty npm install due to babel, autoprefixer/postcss and clean-css, and in particular due to node-sass which is using libsass/node bindings, thus requiring gyp and Python. I'd imagine a leaner setup based on a precompiled sassc binary for SCSS processing could be useful.